Active torque coupling with hydraulically-actuated rotary clutch actuator

ABSTRACT

A torque transfer mechanism having a multi-plate friction clutch connecting a pair of rotary members and an electrohydraulic clutch system for controlling engagement of the friction clutch. The electrohydraulic clutch system can include an actuator with first and second components with a plurality of actuation chambers that are employed to control the position of the second component relative to the first component. Relatively high pressure fluid may be directed through or around one or more bypassed portions of the actuation chambers when relatively low compressive clutch engagement forces are to be exerted on the friction clutch. The bypassed portions of the actuation chambers may be sequentially or progressively operated based on a position of the second component relative to the first component to increase an amount of force that is applied by actuator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/771,664 filed on Feb. 4, 2004.

FIELD OF THE INVENTION

The present invention relates generally to power transfer systems forcontrolling the distribution of drive torque between the front and reardrivelines of a four-wheel drive vehicle and/or the left and rightwheels of an axle assembly. More particularly, the present invention isdirected to a power transmission device for use in motor vehicledriveline applications having a torque transfer mechanism equipped witha clutch actuator that is operable for controlling actuation of amulti-plate friction clutch.

BACKGROUND OF THE INVENTION

In view of increased demand for four-wheel drive vehicles, a plethora ofpower transfer systems are currently being incorporated into vehiculardriveline applications for transferring drive torque to the wheels. Inmany vehicles, a power transmission device is operably installed betweenthe primary and secondary drivelines. Such power transmission devicesare typically equipped with a torque transfer mechanism for selectivelyand/or automatically transferring drive torque from the primarydriveline to the secondary driveline to establish a four-wheel drivemode of operation. For example, the torque transfer mechanism caninclude a dog-type lock-up clutch that can be selectively engaged forrigidly coupling the secondary driveline to the primary driveline toestablish a “part-time” four-wheel drive mode. When the lock-up clutchis released, drive torque is only delivered to the primary driveline forestablishing a two-wheel drive mode.

A modern trend in four-wheel drive motor vehicles is to equip the powertransmission device with an adaptively controlled transfer clutch inplace of the lock-up clutch. The transfer clutch is operable forautomatically directing drive torque to the secondary wheels, withoutany input or action on the part of the vehicle operator, when tractionis lost at the primary wheels for establishing an “on-demand” four-wheeldrive mode. Typically, the transfer clutch includes a multi-plate clutchassembly that is installed between the primary and secondary drivelinesand a clutch actuator for generating a clutch engagement force that isapplied to the clutch assembly. The clutch actuator can be apower-operated device that is actuated in response to electric controlsignals sent from an electronic controller unit (ECU). The electriccontrol signals are typically based on changes in current operatingcharacteristics of the vehicle (i.e., vehicle speed, interaxle speeddifference, acceleration, steering angle, etc.) as detected by varioussensors. Thus, such “on-demand” transfer clutch can utilize adaptivecontrol schemes for automatically controlling torque distribution duringall types of driving and road conditions. Such adaptively controlledtransfer clutches can also be used in association with a centerdifferential operably installed between the primary and secondarydrivelines for automatically controlling interaxle slip and torquebiasing in a full-time four-wheel drive application.

A large number of adaptively controlled transfer clutches have beendeveloped with an electromechanical clutch actuator that can regulatethe amount of drive torque transferred to the secondary driveline as afunction of the electric control signal applied thereto. In someapplications, the transfer clutch employs an electromagnetic clutch asthe power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024discloses a electromagnetic coil that is incrementally activated tocontrol movement of a ball-ramp drive assembly for applying a clutchengagement force to the multi-plate clutch assembly. Likewise, JapaneseLaid-open Patent Application No. 62-18117 discloses a transfer clutchequipped with an electromagnetic clutch actuator for directlycontrolling actuation of the multi-plate clutch pack assembly. As analternative, the transfer clutch can employ an electric motor and amechanical drive assembly as the power-operated clutch actuator. Forexample, U.S. Pat. No. 5,323,871 discloses a transfer clutch equippedwith an electric motor that controls rotation of a sector plate which,in turn, controls pivotal movement of a lever arm that is operable forapplying the clutch engagement force to the multi-plate clutch assembly.Likewise, Japanese Laid-open Patent Application No. 63-66927 discloses atransfer clutch which uses an electric motor to rotate one cam plate ofa ball-ramp operator for engaging the multi-plate clutch assembly.Finally, U.S. Pat. Nos. 4,895,236 and 5,423,235 respectively disclose atransfer clutch having an electric motor which drives a reductiongearset for controlling movement of a ball screw operator and aball-ramp operator which, in turn, apply the clutch engagement force tothe clutch assembly.

In contrast to the electromechanical clutch actuators discussedpreviously, it is also well known to equip the transfer clutch with anelectro-hydraulic clutch actuator. For example, U.S. Pat. Nos. 4,862,769and 5,224,906 generally disclose use of an electric motor or solenoid tocontrol the fluid pressure exerted by an apply piston on a multi-plateclutch assembly. In addition, U.S. Pat. No. 6,520,880 discloses ahydraulic actuation system for controlling the fluid pressure suppliedto a hydraulic motor arranged which is associated with a differentialgear mechanism in a drive axle assembly.

While many adaptive clutch actuation systems similar to those describedabove are currently used in four-wheel drive vehicles, a need exists toadvance the technology and address recognized system limitations. Forexample, the size and weight of the friction clutch components and theelectrical power requirements of the clutch actuator needed to providethe large clutch engagement loads make many systems cost prohibitive foruse in most four-wheel drive vehicle applications. In an effort toaddress these concerns, new technologies are being developed for use inpower-operated clutch actuator applications.

SUMMARY OF THE INVENTION

Thus, its is an objective of the present invention to provide a powertransmission device for use in a motor vehicle having a torque transfermechanism equipped with a unique electrohydraulically-operated clutchactuator that is operable to control adaptive engagement of amulti-plate clutch assembly.

As a related objective of the present invention, the torque transfermechanism is well-suited for use in motor vehicle driveline applicationsto adaptively control the transfer of drive torque between first andsecond rotary members.

According to each preferred embodiment of the present invention, atorque transfer mechanism and an electrohydraulic control system aredisclosed for adaptively controlling the transfer of drive torquebetween first and second rotary members in a power transmission deviceof the type used in motor vehicle driveline applications. The torquetransfer mechanism includes a rotary input member that is configured toreceive drive torque from a source of drive torque, a rotary outputmember adapted to transmit drive torque to an output device and a torquetransmission mechanism that is configured to transfer drive torque fromsaid input member to said output member. The torque transmissionmechanism can include a friction clutch that can be operably disposedbetween the input member and the output member and an electrohydrauliccontrol system for controlling engagement of the friction clutch. Theelectrohydraulic control system can include a clutch actuator, a fluidpump, a motor driving the fluid pump, and a controller. The clutchactuator can include a rotary operator and a thrust mechanism. Therotary operator can have first and second components with a plurality ofactuation chambers and a plurality of return chambers. The firstcomponent can be fixed for rotation with one of the input and outputmembers and the second component can be configured to rotate relative tothe first component. The thrust mechanism can be operable for applying aclutch engagement force on the friction clutch in response to rotationof the second component relative to the first component. The controllercan be operable for controlling the fluid pressure supplied to at leastone of the actuation chambers and the return chambers to thereby controlrotation of the second component relative to the first component of therotary operator. The electrohydraulic control system is configured sothat the actuation chambers are divided into at least two groups. Afirst group is configured such that the activation chamber or activationchambers included therein contain a relatively high pressure fluid whenthe second component is rotated relative to the first component in anactuating direction regardless of a relative position of the first andsecond components. A second group is configured such that the activationchamber or activation chambers included therein contain a relatively lowpressure fluid when the second component is positioned relative to thefirst component within a predetermined actuating interval and containthe relatively high pressure fluid when the second component is rotatedrelative to the first component in the actuation direction by an amountthat exceeds the predetermined actuating interval.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objectives, features and advantages of the present inventionwill become apparent to those skilled in the art from analysis of thefollowing written description, the appended claims, and the accompanyingdrawings in which:

FIG. 1 illustrates the drivetrain of a four-wheel drive vehicle equippedwith a power transmission device according to the present invention;

FIG. 2 is a schematic illustration of the power transmission deviceequipped with a torque transfer mechanism embodying the inventiveconcepts of the present invention;

FIGS. 3 and 3A are sectional views of the torque transfer mechanismconstructed in accordance with a preferred embodiment of the presentinvention;

FIG. 4 is a sectional view of the rotary operator associated with thetorque transfer mechanism shown in FIGS. 3 and 3A;

FIG. 5 is a schematic diagram of the electrohydraulic control circuitassociated with the torque transfer mechanism of the present invention;

FIGS. 6 through 8 are schematic diagrams of alternative electrohydrauliccontrol systems adapted for use with the torque transfer mechanism ofthe present invention;

FIG. 9 is a schematic illustration of an alternative power transmissiondevice available for use with the drivetrain shown in FIG. 1;

FIG. 10 is a schematic illustration of another alternative embodiment ofa power transmission device according to the present invention;

FIG. 11 illustrates an alternative drivetrain arrangement for afour-wheel drive motor vehicle equipped with another power transmissiondevice embodying the present invention;

FIGS. 12 through 15 schematically illustrate different embodiments ofthe power transmission device shown in FIG. 11;

FIG. 16 is an illustration of another drivetrain arrangement for afour-wheel drive vehicle equipped with a power transmission deviceembodying the present invention;

FIG. 17 is a schematic illustration of an alternative construction forthe power transmission device shown in FIG. 16;

FIG. 18 is a sectional view of the torque transfer mechanism constructedin accordance with the teachings of the present invention;

FIG. 19 is a schematic illustration of the power transmission deviceequipped with the torque transfer mechanism of FIG. 18;

FIG. 20 is a perspective view of a portion of the torque transfermechanism of FIG. 18 illustrating the clutch actuator in greater detail;and

FIG. 21 is a sectional view of a portion of the torque transfermechanism of FIG. 18 illustrating the construction of the reaction ringand the actuator ring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a torque transfer mechanism thatcan be adaptively controlled for modulating the torque transferred froma first rotary member to a second rotary member. The torque transfermechanism finds particular application in power transmission devices foruse in motor vehicle drivelines such as, for example, a torque transferclutch in a transfer case, a power take-off unit or an in-line torquecoupling, a torque biasing clutch associated with a differential unit infull-time transfer cases or power take-off unit or in a drive axleassembly, or any other possible torque transfer application. Thus, whilethe present invention is hereinafter described in association withparticular power transmission devices for use in specific drivelineapplications, it will be understood that the arrangements shown anddescribed are merely intended to illustrate embodiments of the presentinvention.

With reference to FIG. 1, a schematic layout of a vehicular drivetrain10 is shown to include a powertrain 12, a first or primary driveline 14driven by powertrain 12, and a second or secondary driveline 16.Powertrain 12 includes an engine 18 and a multi-speed transaxle 20arranged to normally provide motive power (i.e., drive torque) to a pairof first wheels 22 associated with primary driveline 14. Primarydriveline 14 further includes a pair of axleshafts 24 connecting wheels22 to a differential unit 25 associated with transaxle 20. Secondarydriveline 16 includes a power take-off unit (PTU) 26 driven by theoutput of transaxle 20, a propshaft 28 driven by PTU 26, a pair ofaxleshafts 30 connected to a pair of second wheels 32, and a drive axleassembly 34 that is operable to selectively transfer drive torque frompropshaft 28 to axle halfshafts 30.

Drive axle assembly 34 is a power transmission device according to onepreferred embodiment of the present invention. In particular, drive axleassembly 34 is shown schematically in FIG. 2 to include a housingassembly 36 which encloses a torque transfer mechanism 38 and adifferential unit 40. Torque transfer mechanism 38 functions toselectively transfer drive torque from propshaft 28 to an inputcomponent of differential unit 40. Specifically, the torque transfermechanism, hereinafter referred to as torque coupling 38, includes aninput shaft 42 driven by propshaft 28, a pinion shaft 44, a transferclutch 46 operably connected between input shaft 42 and pinion shaft 44,and a clutch actuator 48 for engaging transfer clutch 46.

With continued reference to the drawings, drivetrain 10 is shown tofurther include an electronically-controlled power transfer system forpermitting a vehicle operator to select between a two-wheel drive mode,a locked (“part-time”) four-wheel drive mode, and an adaptive(“on-demand”) four-wheel drive mode. In this regard, transfer clutch 46can be selectively engaged for transferring drive torque from inputshaft 42 to pinion shaft 44 for establishing both of the part-time andon-demand four-wheel drive modes. The power transfer system includes anelectrohydraulic control system 50 for selectively actuating clutchactuator 48, vehicle sensors 52 for detecting certain dynamic andoperational characteristics of the motor vehicle, a mode selectmechanism 54 for permitting the vehicle operator to select one of theavailable drive modes, and an electronic control unit (ECU) 56 forcontrolling operation of the components associated with electrohydrauliccontrol system 50 which, in turn, controls actuation of clutch actuator48 in response to input signals from vehicle sensors 52 and modeselector 54.

Referring primarily to FIGS. 2 and 3, transfer clutch 46 generallyincludes a first clutch member 60 driven by input shaft 42, a secondclutch member 62 driving pinion shaft 44, and a multi-plate clutch pack64 of alternately interleaved clutch plates installed between inputshaft 42 and pinion shaft 44. As shown in this particular arrangement,first clutch member 60 is a clutch drum fixed for rotation with inputshaft 42 and second clutch member 62 is a clutch hub fixed (i.e.,splined) for rotation with pinion shaft 44. Pinion shaft 44 drives apinion gear 66 meshed with a ring gear 68 which, in turn, drivesdifferential unit 40. Differential unit 40 includes a carrier 70 drivenby ring gear 68, a pair of pinion gears 72 rotatably supported on pinionposts 74 fixed to carrier 70, and a pair of side gears 76. Side gears 76are meshed with both pinion gears 74 and are coupled for rotation with acorresponding one of axleshafts 30.

Referring now to FIGS. 3 and 3A, a preferred construction for torquecoupling 38 will now be described in greater detail. Torque coupling 38includes a case assembly 78 that is mounted in or forms part of housing68 of drive axle assembly 34. A bearing assembly 80 supports input shaft42 for rotation relative to case assembly 78. In addition, input shaft42 is shown to include an integral end plate 82 that is rigidly secured(i.e., welded) to clutch drum 60. An end plate segment 84 of drum 60 isrotatively supported by a bearing assembly 86 from case assembly 78.Pinion shaft 44 has a first end rotatably supported by a bushing orbearing assembly 88 in a central bore formed in input shaft 42 while itssecond end extends out of end plate segment 84 of drum 60 and isrotatably supported from case assembly 78 by a bearing assembly 90.Clutch actuator 48 generally includes a rotary operator 92, a thrustmechanism 94, and an apply plate 96. Apply plate 96 is secured (i.e.,splined) for rotation with drum 60 of transfer clutch 46.

As will be detailed, clutch actuator 48 is operable for generating andexerting a compressive clutch engagement force on clutch pack 64. Suchengagement of clutch pack 64 causes rotary power (“drive torque”) to betransferred from input shaft 42 to pinion shaft 44. Specifically, clutchactuator 48 is operable for controlling axial movement of apply plate 96and thus, the magnitude of the clutch engagement force applied to clutchpack 64. In particular, apply plate 96 is axially moveable relative toclutch pack 64 between a first or “released” position and a second or“locked” position. With apply plate 96 in its released position, aminimum clutch engagement force is exerted on clutch pack 64 such thatvirtually no drive torque is transferred from input shaft 42 throughtransfer clutch 46 to pinion shaft 44, thereby establishing thetwo-wheel drive mode. In contrast, movement of apply plate 96 to itslocked position causes a maximum clutch engagement force to be appliedto clutch pack 64 such that pinion shaft 44 is, in effect, coupled forcommon rotation with input shaft 42, thereby establishing part-timefour-wheel drive mode. Accordingly, controlling the position of applyplate 96 between its released and locked positions permits adaptiveregulation of the amount of drive torque transferred from input shaft 42to pinion shaft 44, thereby establishing the on-demand four-wheel drivemode.

Rotary operator 92 includes a first or reaction ring 100 that isconcentrically aligned with a second or “actuator” ring 102. The ringsare retained in an annular chamber for 104 defined between end plate 82and a retainer plate 106. While not shown, retainer plate 106 is securedby a plurality of bolts to end plate 82 which also extend throughmounting holes 108 in reaction ring 100. As such, reaction ring 100 isfixed for common rotation with input shaft 42.

As best seen from FIG. 4, reaction ring 100 includes a cylindrical bodysegment 110 and a plurality of radially inwardly projecting lugs 112which define a complimentary number of longitudinally extending channels114 therebetween. Likewise, actuator ring 102 has a cylindrical bodysegment 116 and a plurality of separator plates 118 that are retained inlongitudinal slots 120 formed in body segment 116. An outer edge surface122 of each separator plate 118 is aligned to be in sliding engagementwith an inner wall surface 124 of channels 114. A seal strip 126 may beattached to edge surface 122 to provide a sliding fluid-tight interfacebetween separator plates 118 and body segment 110 of reaction ring 100.Likewise, a terminal edge surface 128 of each lug 112 is aligned to bein sliding engagement with an outer wall surface 130 of actuator ring102. Each separator plate 118 defines an actuation chamber 132 and areturn chamber 134 between adjacent pairs of lugs 112. As such, aplurality of circumferentially-spaced alternating actuation chambers 132and return chambers 134 are established in association with rotaryoperator 92.

To provide means for supplying hydraulic fluid from electrohydrauliccontrol system 50 to actuation chambers 132, a first flow path is formedin input shaft 42 and its end plate segment 82. The first flow passageincludes an annular chamber 140 which communicates with a series ofcircumferentially-spaced flow passages 142 having ports 144 in fluidcommunication with actuation chambers 132. Similarly, a second flow pathprovides means for supplying hydraulic fluid from control system 50 toreturn chambers 134. This second flow path includes an annular chamber146 which communicates with a series of circumferentially-spaced flowpassages 148 having ports 150 in fluid communication with returnchambers 134. As will be detailed, increasing the fluid pressuredelivered through ports 144 to actuation chambers 132 while decreasingthe fluid pressure delivered through ports 150 to return chambers 134causes actuator ring 102 to move (i.e., index) in a first rotarydirection (i.e., counterclockwise) relative to reaction ring 100 forcausing thrust mechanism 94 to urge apply plate 96 to move toward itslocked position. In contrast, decreasing the fluid pressure in actuationchambers 132 and increasing the fluid pressure in return chambers 134causes actuator ring 102 to index in a second rotary direction (i.e.,clockwise) relative to reaction ring 100 for causing thrust mechanism 94to permit apply plate 96 to move toward its released position.

With continued reference primarily to FIG. 3A, thrust mechanism 94 isshown to be a ball ramp unit having a first cam member 152, a second cammember 154, and rollers, such as balls 156. First cam member 152 isfixed via a spline connection 158 for rotary movement with actuator ring102. Likewise, second cam member 154 is fixed to apply plate 96 forcommon rotation with drum 60 and input shaft 42. Each ball 156 isdisposed in a cam channel defined between a cam track 170 formed infirst cam member 152 and a corresponding cam track 172 formed in secondcam member 154. Preferably, a plurality of cam channels are providedbetween first cam member 152 and second cam member 154 with cam tracks170 and 172 configured as oblique sections of a helical torus. However,balls 156 and cam tracks 170 and 172 may be replaced with alternativecam components and/or ramp configurations that function to cause axialdisplacement of second cam member 154. In any arrangement, the loadtransferring components can not be self-locking or self-engaging so asto permit fine control of the translational movement of apply plate 96(via second cam member 154) for precise control of the engagementcharacteristics (i.e., torque transfer) of transfer clutch 46. As seen,a thrust bearing 176 is located between end plate 82 and first cammember 152.

Ball ramp unit 94 further includes a torsional return spring 178 that isoperably disposed between cam members 152 and 154. Return spring 178functions to angularly bias cam members 152 and 154 to return to a“retracted” position for de-energizing ball ramp unit 94. Such angularmovement of the cam members to the retracted position due to the biasingof return spring 178 results in angular movement of actuator ring 102relative to reaction ring 102 in the second angular direction toward afirst or “low pressure” position and translational movement of applyplate 96 toward its released position. With actuator ring 102 in its lowpressure position (as shown in FIG. 4), ball ramp unit 94 isde-energized and apply plate 96 is in its released position so as toexert a predetermined minimum clutch engagement force on clutch pack 64for releasing engagement of transfer clutch 46.

Electrohydraulic control system 50 is operable to supply high pressurefluid to actuation chambers 132 for causing actuator ring 102 to rotaterelative to reaction ring 100 in the first direction from its lowpressure position toward a second or “high pressure” position. Suchmovement of actuator ring 102 results in corresponding relative angularmovement between cam members 152 and 154 from the retracted positiontoward a second or “extended” position for energizing ball ramp unit 94.Accordingly, the profile of cam tracks 170 and 172 establishes theresultant amount of translational movement of second cam member 154required to cause corresponding axial movement of apply plate 96 fromits released position toward its locked position. When actuator ring 102is in its high pressure position, ball ramp unit 94 is fully energizedand apply plate 96 is located in its locked position such that themaximum clutch engagement force is exerted on clutch pack 64 for fullyengaging transfer clutch 46. Furthermore, electrohydraulic controlsystem 50 is operable to supply high pressure fluid to return chambers134 and vent actuation chambers 132 for causing actuator ring 102 torotate relative to reaction ring 100 in the second direction from itshigh pressure position. Such angular movement of actuator ring 102results in corresponding relative angular movement between cam members152 and 154 from the extended position toward the retracted position forreleasing ball ramp unit 94. As such, apply plate 96 is caused to movefrom its locked position toward its released position for releasingengagement of transfer clutch 46.

With apply plate 96 in its released position, virtually no drive torqueis transferred from input shaft 42 to pinion shaft 44 through torquecoupling 38 so as to effectively establish the two-wheel drive mode. Incontrast, location of apply plate 96 in its locked position results in amaximum amount of drive torque being transferred to pinion shaft 44 forcoupling pinion shaft 44 for common rotation with input shaft 42,thereby establishing the part-time four-wheel drive mode. Accordingly,controlling the position of apply plate 96 between its released andlocked positions permits variable control of the amount of drive torquetransferred from input shaft 42 to pinion shaft 44 for establishing anon-demand four-wheel drive mode. Thus, the magnitude of the fluidpressure supplied to actuation chambers 132 and return chambers 134controls the angular position of actuator ring 102 relative to reactionring 100, thereby controlling actuation of ball ramp unit 94 and theresulting movement of apply plate 96 between its released and lockedpositions.

Referring to FIGS. 3 and 5, the components associated withelectrohydraulic control system 50 will now be described. The basiccomponents of control system 50 include a fluid pump 190 operable todraw fluid from a fluid source or sump 192 within casing 78, an electricmotor 194 driving pump 190, an electronic variable orifice (EVO) valve196, and a directional control valve 198. Control system 50 furtherincludes a temperature sensor 200, a pressure sensor 202 and aspring-type pressure relief valve (PRV) 204. Motor 194 is a low powerunit operable for causing pump 190 to draw fluid from sump 192 through afirst flow path 206. The magnitude of the line pressure discharged frompump 190 is delivered to an inlet of EVO valve 196 through a second flowpath 208. However, PRV 204 functions to limit the line pressuredelivered to the inlet of EVO valve 196 to a predefined maximum fluidpressure value. EVO valve 196 includes a moveable valve element thatfunctions to vary the line pressure to a “control” pressure value thatis supplied from an outlet of EVO valve 196 to an inlet of control valve198 via a third flow path 210. As seen, the low pressure fluid by-passedthrough EVO valve 196 flows through a fourth flow path 212 and is usedto cool and lubricate clutch pack 64. Fluid discharged from controlvalve 198 is returned via a fifth flow path 214 to sump 192. Finally,control valve 198 is shown to be in fluid communication with actuationchambers 132 via a sixth flow path 216 and in fluid communication withreturn chambers 134 via a seventh flow path 218. It will be appreciatedthat the components of the electrohydraulic control system can beintegrated into case assembly 78 to define a stand-alone assembly or, inthe alternative, be remotely located and connected via suitable hosesand tubing.

Rotary operator 92 is partially shown in FIG. 5 to illustrate itshydraulic connections with control valve 198. In the arrangement shown,EVO valve 196 is preferably an electromagnetic flow control valve thatis capable of variably regulating the fluid pressure delivered tocontrol valve 198. Likewise, control valve 198 is shown as a 3-positionshuttle valve. As seen, ECU 56 is operable to send electrical controlsignals to electric motor 194, EVO valve 196 and control valve 198. EVOvalve 196 is selectively actuated by ECU 56 to control the fluidpressure supplied to third flow path 210 while control valve 198 isselectively actuated to control delivery of the pressurized fluid toeither of actuation chambers 132 or return chambers 134. As best seenfrom FIGS. 3 and 3A, sixth flow path 216 is in fluid communication withannular chamber 140 via a flow passage 220 formed in case 78 whileseventh flow path 218 communicates with annular chamber 146 via anotherflow passage 222. A plurality of ring seals 224 are provided betweencase 78 and input shaft 42 to delimit chambers 140 and 146.

As contemplated by the present invention, ECU 56 is programmed toaccurately control the angular position of actuator ring 102 relative toreaction ring 100 based on a predefined control strategy fortransferring the desired amount of drive torque across transfer clutch46. The control strategy functions to determine and generate theelectric control signals sent to EVO valve 196 and control valve 198based on the mode signal from mode selector 54 and the sensor signalsfrom sensors 52. In addition, pressure sensor 202 sends a signal to ECU56 that is indicative of the fluid pressure delivered through sixth flowpath 216 to actuation chambers 132. Likewise, temperature sensor 200sends a signal to ECU 56 that is indicative of the fluid temperature insump 192.

In operation, if the vehicle operator selects the two-wheel drive mode,EVO valve 196 is opened and control valve 198 is initially actuated tocause third flow path 210 to be placed in communication with seventhflow path 218 while sixth flow path 216 is placed in communication withfifth flow path 214, whereby actuation chambers 132 are vented to sump192 while fluid at control pressure is supplied to return chambers 134.This results in actuator ring 102 being forced to its low pressureposition relative to reaction ring 100 for releasing engagement oftransfer clutch 46. Thereafter, control valve 198 can be shifted intoits closed position with no fluid delivered to actuation chambers 132 orreturn chambers 134 since return spring 178 forcibly biases actuatorring 102 to remain in its low pressure position. In contrast, uponselection of the part-time four-wheel drive mode, EVO valve 196 isopened and control valve 198 is actuated to connect third flow path 210to sixth flow path 216 and also connect seventh flow path 218 to fifthflow path 214, whereby actuation chambers 132 are supplied with fluid atcontrol pressure and return chambers 134 are vented to sump 192. Thehigh pressure fluid supplied to actuation chambers 132 causes actuatorring 102 to move to its high pressure position relative to reaction ring100, whereby ball ramp unit 94 is fully energized for moving apply plate96 to its locked position for fully engaging transfer clutch 46. PRV 204functions to limit the maximum fluid pressure that can be delivered toactuation chambers 132 during part-time four-wheel drive operation,thereby providing a torque limiting feature to prevent damage to clutchpack 64.

When mode selector 54 indicates selection of the on-demand four-wheeldrive mode, ECU 56 actuates control valve 198 so as to connect thirdflow path 210 to sixth flow path 216 while also connecting seventh flowpath 218 to fifth flow path 214. As such, return chambers 134 are ventedand supply actuation chambers 132 are supplied with pressurized fluidfrom EVO valve 196. ECU 56 functions to adaptively control EVO valve 196so as to initially supply fluid at a predetermined relatively lowpressure to actuation chambers 132 that causes actuator ring 102 toindex slightly relative to reaction ring 100 in the first direction.This angular movements causes actuator ring 102 to move from its lowpressure position to an intermediate or “ready” position which, in turn,results in ball ramp unit 94 moving apply plate 96 from its releasedposition to a “stand-by” position. In the stand-by position, apply plate96 exerts a small clutch engagement force on clutch pack 64.Accordingly, a small amount of drive torque is delivered to pinion shaft44 through transfer clutch 46 in this adapt-ready condition. Thereafter,ECU 56 determines when and how much drive torque needs to be transferredto pinion shaft 44 based on the current tractive conditions and/oroperating characteristics of the motor vehicle, as detected by sensors52.

Sensors 52 detect such parameters as, for example, the rotary speed ofthe input and pinion shafts, the vehicle speed and/or acceleration, thetransmission gear, the on/off status of the brakes, the steering angle,the road conditions, etc. Such sensor signals are used by ECU 56 todetermine a desired output torque value utilizing a control logic schemethat is incorporated into ECU 56. In particular, the control logiccorrelates the desired torque value to a fluid pressure value to bedelivered to actuation chambers 132. Based on this desired pressurevalue, ECU 56 actively controls actuation of EVO valve 196 to generate acorresponding pressure level in actuation chambers 132. Pressure sensor202 provides ECU 56 with direct feedback as to the actual fluid pressurein actuation chambers 132 so as to permit precise control of clutchactuator 52.

In addition to adaptive on-demand torque control, the present inventionpermits automatic release of transfer clutch 46 in the event of an ABSbraking condition or during the occurrence of an over-temperaturecondition. Specifically, when ECU 56 is signaled that an ABS brakecondition occurs, control valve 198 is immediately actuated to connectreturn chambers 134 with EVO valve 196 while actuation chambers 132 arevented to sump 192. Also, EVO valve 196 is fully opened to send fullpressure to return chambers 134, thereby forcibly moving actuator ring102 in its second direction to its low pressure position for fullyreleasing transfer clutch 46. Moreover, if temperature sensor 200detects that the fluid temperature in sump 192 exceeds a predeterminedthreshold value, the same type of immediate release of transfer clutch46 will be commanded by ECU 56.

While the control scheme has been described based on an on-demand torquetransfer strategy, it is contemplated that a differential “mimic”control strategy can likewise be used. Specifically, the torquedistribution between input shaft 42 and pinion shaft 44 can becontrolled to normally maintain a predetermined rear/front ratio (i.e.,70:30, 50:50, etc.) so as to simulate the inter-axle torque splittingfeature typically provided by a center differential unit. This desiredtorque distribution can then be adaptively controlled to address losttraction at either set of wheels. Regardless of whether the controllogic scheme is based on an on-demand or a differential torque transferstrategy, accurate control of the fluid pressure delivered to actuationchambers 132 of rotary operator 92 provides the desired torque transfercharacteristics being established across transfer clutch 46.

It is contemplated that the 3-position directional control valve 198shown in FIG. 5 could easily be substituted with a 2-positiondirectional valve or any other known valve device capable of controllingthe direction of flow in the hydraulic circuit. Likewise, FIG. 6illustrates a modified arrangement for electrohydraulic control system50 where control valve 198 now receives the line pressure directly frompump 190 via flow path 208 and EVO valve 196 is functional to regulatethe fluid pressure within actuation chambers 132 by controlling thefluid pressure within flow path 216. Again, EVO valve 196 is normallyopen with the by-passed fluid delivered through flow path 212 to cooland lubricate clutch pack 64. In operation, the fluid pressure suppliedto actuation chambers 132 is increased by closing EVO valve 196. Assuch, variable control of EVO valve 156 again results in variablepressure control within actuation chambers 132.

FIG. 7 is a hydraulic schematic of an electrohydraulic clutch system 50which is generally similar to FIG. 6 except that control valve 198 hasbeen eliminated. As such, ECU 56 controls the pumping direction of pump190 by reversing the polarity of motor 194. Check valves 230 and 232 areprovided to permit such a bidirectional pumping action. As before, thepressure profile of the fluid delivered to actuation chambers 132 isadaptively controlled via variable actuation of EVO valve 196.Optionally, EVO valve 196 can be eliminated with the pressure profilewithin actuation chambers 132 and return chambers 134 directlycontrolled by variable control of the rotary speed (RPM) and directionof electric motor 194.

The previous hydraulic circuits utilized control valve 198 fordirectional control in conjunction with EVO valve 196 for adaptivepressure regulation. As an alternative, FIG. 8 illustrates a hydrauliccircuit wherein a proportional control valve 234 is used to regulateboth the directional and pressure characteristics. Preferably,proportional control valve 234 is a pulse width modulated (PWM) valvehaving a moveable element that is controlled by an electromagneticsolenoid based on electric control signals from ECU 56. In addition, aflow control valve 236 is provided in flow path 208 to preventdead-heading of pump 190 and to provide cooling flow to clutch pack 64.

The arrangement shown for drive axle assembly 34 of FIG. 2 is operableto provide the on-demand four-wheel drive mode by adaptivelytransferring drive torque from primary driveline 14 (via propshaft 28)to secondary driveline (via pinion shaft 44). In contrast, a drive axleassembly 34A is shown in FIG. 9 with torque coupling 38 now installedbetween differential case 70 and one of axleshafts 30 to provide anadaptive system for biasing the torque and limiting intra-axle slipbetween the rear wheels 32. As before, torque coupling 38 isschematically shown to include transfer clutch 46 and clutch actuator48, the construction and function of which are understood to be similarto the detailed description previously provided herein for eachsub-assembly. It will be understood that this particular “limited slip”differential arrangement can either be used in association with theon-demand drive axle assembly shown in FIG. 2 or in association with adrive axle assembly wherein propshaft 28 directly drives differentialunit 40.

Referring now to FIG. 10, a drive axle assembly 34B is schematicallyshown to include a pair of torque couplings 38L and 38R that areoperably installed between a driven shaft 44 or 28 and axleshafts 30.The driven pinion shaft drives a right-angled gearset including pinion66 and ring gear 68 which, in turn, drives a transfer shaft 240. Firsttorque coupling 38L is shown disposed between transfer shaft 240 and theleft axleshafts 30 while second torque coupling 38R is disposed betweentransfer shaft 240 and the right axle shaft 30. Each coupling includes acorresponding transfer clutch 46L, 46R and a clutch actuator 48L, 48R.Accordingly, independent slip control between the driven pinion shaftand each wheel 32 is provided by this arrangement. A common sump isprovided with ECU 56 controlling independent actuation of both clutchactuators 48L and 48R.

To illustrate an alternative power transmission device to which thepresent invention is applicable, FIG. 11 schematically depicts afront-wheel based four-wheel drive drivetrain layout 10′ for a motorvehicle. In particular, engine 18 drives a multi-speed transaxle 20having an integrated front differential unit 25 for driving front wheels22 via axle shafts 24. As before, PTU 26 is also driven by transaxle 20for delivering drive torque to the input member of a torque coupling238. The output member of torque coupling 238 is coupled to propshaft 28which, in turn, drives rear wheels 32 via axle assembly 34. Rear axleassembly 34 can be a traditional driven axle with a differential or, inthe alternative, be similar to the arrangements described in associationwith FIGS. 9 or 10. Accordingly, in response to the detection of a frontwheel slip condition, torque coupling unit 238 is adaptively actuated todeliver drive torque “on-demand” to rear wheels 32. Again, it iscontemplated that torque coupling unit 238 is substantially similar instructure and function to that of torque coupling unit 38 previouslydescribed herein.

Referring now to FIG. 12, torque coupling 238 is schematicallyillustrated in association with an on-demand four-wheel drive systembased on a front-wheel drive vehicle similar to that shown in FIG. 11.In particular, an output shaft 240 of transaxle 20 is shown to drive anoutput gear 242 which, in turn, drives an input gear 244 that is fixedto a carrier 246 associated with front differential unit 25. To providedrive torque to front wheels 22, front differential unit 25 includes apair of side gears 248 that are connected to front wheels 22 viaaxleshafts 24. Differential unit 25 also includes pinions 250 that arerotatably supported on pinion shafts fixed to carrier 246 and which aremeshed with side gears 348. A transfer shaft 252 is provided fortransferring drive torque from carrier 246 to a clutch hub 62 associatedwith transfer clutch 46. PTU 26 is a right-angled drive mechanismincluding a ring gear 324 fixed for rotation with drum 60 of transferclutch 46 and which is meshed with a pinion gear 256 fixed for rotationwith propshaft 28. According to the present invention, the componentsschematically shown for torque transfer mechanism 238 are understood tobe similar to those previously described. In operation, the powertransfer system permits drive torque to be adaptively transferred fromthe primary (i.e., front) driveline to the secondary (i.e., rear)driveline.

Referring to FIG. 13, a modified version of the power transmissiondevice shown in FIG. 12 now includes a second torque coupling 238A thatis arranged to provide a limited slip feature in association withprimary differential 25. As before, torque coupling 238 provideson-demand transfer of drive torque from the primary driveline to thesecondary driveline. In addition, second torque coupling 238A nowprovides on-demand torque biasing (side-to-side) between axleshafts 24of primary driveline 14.

FIG. 14 illustrates another modified version of FIG. 12 wherein anon-demand four-wheel drive system is shown based on a rear-wheel drivemotor vehicle that is arranged to normally deliver drive torque to rearwheels 32 while selectively transmitting drive torque to front wheels 22through a torque coupling 238. In this arrangement, drive torque istransmitted directly from transmission output shaft 240 to powertransfer unit 26 via a drive shaft 260 which interconnects input gear244 to ring gear 254. To provide drive torque to front wheels 22, torquecoupling 238 is shown operably disposed between drive shaft 260 andtransfer shaft 252. In particular, transfer clutch 46 is arranged suchthat drum 60 is driven with ring gear 254 by drive shaft 260. As such,clutch actuator 48 can be adaptively actuated to transfer drive torquefrom drum 60 through clutch pack 64 to hub 62 which, in turn, drivescarrier 246 of front differential unit 25 via transfer shaft 252.

In addition to the on-demand four-wheel drive systems shown previously,the power transmission technology of the present invention can likewisebe used in full-time four-wheel drive systems to adaptively bias thetorque distribution transmitted by a center or “interaxle” differentialunit to the front and rear drivelines. For example, FIG. 15schematically illustrates a full-time four-wheel drive system which isgenerally similar to the on-demand four-wheel drive system shown in FIG.14 with the exception that an interaxle differential unit 270 is nowoperably installed between carrier 246 of front differential unit 25 andtransfer shaft 252. In particular, output gear 244 is fixed for rotationwith a carrier 272 of interaxle differential 270 from which pinion gears274 are rotatably supported. A first side gear 276 is meshed with piniongears 274 and is fixed for rotation with drive shaft 260 so as to bedrivingly interconnected to the rear driveline through power transferunit 26. Likewise, a second side gear 278 is meshed with pinion gears274 and is fixed for rotation with carrier 246 of front differentialunit 25 so as to be drivingly interconnected to the front driveline.Torque transfer mechanism 238 is now shown to be operably disposedbetween side gears 276 and 278. Torque transfer mechanism 238 isoperably arranged between the driven outputs of interaxle differential270 for providing an adaptive torque biasing and slip limiting function.

Referring now to FIG. 16, a drivetrain for a four-wheel drive vehicle isshown to include engine 18, a multi-speed transmission 20′ fordelivering drive torque to a primary or rear driveline 16′ through apower transmission device, thereinafter referred to as transfer case300. As seen, transfer case 300 has a rear output shaft 302interconnected between the output of transmission 20′ and a rearpropshaft 28′. Further, propshaft 28′ is shown to drive a pinion shaft44′ for driving a drive axle assembly which, in this example, is similarto rear axle assembly 34A of FIG. 9. A secondary or front driveline 14′includes a front propshaft 304 interconnecting a front output shaft 306of transfer case 300 to a conventional front axle assembly 308. Atransfer assembly associated with transfer case 300 includes a firstsprocket 310 rotatably supported on rear output shaft 302, a secondsprocket 312 fixed to front output shaft 306, and a chain 314 enmeshedtherebetween. Transfer case 300 is shown to include a torque coupling 38for providing on-demand transfer of drive torque from rear output shaft302 through the transfer assembly to first output shaft 306. As seen,transfer case 300 has an electrohydraulic control system 50′ that iscontrolled by ECU 56 in coordination with electrohydraulic controlsystem 50 associated with torque coupling 38 in drive axle 34A.

Referring now to FIG. 17, a full-time 4 WD system is shown to include atransfer case 300′ which is generally similar to transfer case 300 ofFIG. 16 except that an interaxle differential 320 is provided between aninput shaft 322 and output shafts 302 and 306. As is conventional, inputshaft 322 is driven by the output of transmission 20′. Differential 320includes an input defined as a planet carrier 324, a first outputdefined as a first sun gear 326, a second output defined as a second sungear 328, and a gearset for permitting speed differentiation betweenfirst and second sun gears 326 and 328. The gearset includes a pluralityof meshed pairs of first planet gears 330 and second pinions 332 whichare rotatably supported by carrier 324. First planet gears 330 are shownto mesh with first sun gear 326 while second planet gears 332 are meshedwith second sun gear 328. First sun gear 326 is fixed for rotation withrear output shaft 302 so as to transmit drive torque to rear driveline16′. To transmit drive torque to front driveline 14′, second sun gear328 is coupled to the transfer assembly which again includes firstsprocket 310 rotatably supported on rear output shaft 302, secondsprocket 312 fixed to front output shaft 306, and power chain 314.

While the electrohydraulic control system 50 has been described andillustrated thus far as having a clutch actuator 48 with an actuatorring 102 that is subjected to uniformly increasing fluid pressures toeffect movement of the actuator ring 102 in a rotational direction thattends to increase the compressive clutch engagement force that isexerted on the clutch pack 64, it will be appreciated that theinvention, in its broadest aspects, may be constructed somewhatdifferently. For example, the rotary operator 92 a may be constructed asshown in FIGS. 18 and 19. In this example, the electrohydraulic controlsystem 50 a can be configured so that the torque that is produced by theclutch actuator 48 a can be varied based on the position of the actuatorring 102 a relative to the reaction ring 100 a.

With reference to FIGS. 19 through 21, the reaction ring 100 a can begenerally similar to the reaction ring 100 that is illustrated in FIG.4. The projecting lugs 112 a can be configured to sealingly engage anouter portion 1000 of the hub 1002 of the actuator ring 102 a. In theparticular example provided, a seal member 1004 is housed in eachprojecting lug 112 a and is configured to form a seal between thereaction ring 100 a and the actuator ring 102 a as will be described indetail, below. Optionally, one or more hydraulic valves 1008 can beemployed to cause the hydraulic pressure that acts on one or more groupsof the actuation chambers 132 a to be phased relative to other groups ofthe actuation chambers 132 a according to the relative position of thereaction ring 100 a and the actuator ring 102 a.

The hydraulic valves 1008 can include an associated one of the supplyports 144 a and a fluid distribution feature 1010 that will be discussedin greater detail below. One or more of the ports 144 a that providepressurized fluid to the actuation chambers 132 a can be spaced orphased so as to provide pressurized fluid an associated one of theactuation chambers 132 a when the actuator ring 102 a has been rotatedto a predetermined location relative to the reaction ring 100 a. Forexample, the ports 144 a-1 through 144 a-3 are disposed betweenrespective lugs 112 a and separator plates 118 a (e.g., port 144 a-1 isdisposed between lug 112 a-1 and separator plate 118 a-1) when theclutch actuator 48 a outputs a minimum compressive clutch engagementforce onto the clutch pack 64 a and each of the remaining ports 144 aare spaced progressively farther from this relative position or datum.In the particular example provided:

-   -   the port 144 a-4 is spaced by an additional first predetermined        offset interval 1020-4, such as 4°, relative to a predetermined        datum, such as datum 1022-4, to provide pressurized fluid to the        activation chamber 132 a-4 after the actuator ring 102 a has        rotated through an angle that is about equal to the first        predetermined offset interval;    -   the port 144 a-5 is spaced by an additional second predetermined        offset interval 1020-5, such as 8°, relative to a predetermined        datum, such as datum 1022-5, to provide pressurized fluid to the        activation chamber 132 a-5 after the actuator ring 102 a has        rotated through an angle that is about equal to the second        predetermined offset interval;    -   the port 144 a-6 is spaced by an additional third predetermined        offset interval 1020-6 relative to a predetermined datum, such        as datum 1022-6, to provide pressurized fluid to the activation        chamber 132 a-6 after the actuator ring 102 a has rotated        through an angle that is about equal to the third predetermined        offset interval; and    -   the port 144 a-7 is spaced by an additional fourth predetermined        offset interval 1020-7 (relative to a predetermined datum, such        as datum 1022-7), such as 16°, to provide pressurized fluid to        the activation chamber 132 a-7 after the actuator ring 102 a has        rotated through an angle that is about equal to the fourth        predetermined offset interval.

The actuator ring 102 a can be generally similar to that which isdescribed above in conjunction with the embodiment of FIG. 4, exceptthat the separator plates 118 a can be integrally formed with theremainder of the actuator ring 102 a and the fluid distribution feature1010, such as a groove, can be formed in the hub 1002 of the actuatorring 102 a, and one or more of the separator plates 118 a-4 through 118a-7 can be configured to selectively direct pressurized fluid from anassociated supply port 144 a to an associated return port 150 a (e.g.,supply port 144 a-4 can selectively supply fluid pressure directly toreturn port 150 a-4).

Each fluid distribution feature 1010 can be configured to permit fluidcommunication between corresponding supply ports 144 a and return ports150 a when the actuator ring 102 a has not rotated relative to thereaction ring 100 a by a corresponding actuating interval. The actuatinginterval may be any appropriate interval but in the example provided,each actuating interval corresponds to an associated one of thepredetermined offset intervals (i.e., the actuating interval associatedwith the fluid distribution feature 1010-4 can extend over approximately4° of the circumference of the hub 1002 of the actuator ring 102 a, theactuating interval associated with the fluid distribution feature 1010-5can extend over approximately 8° of the circumference of the hub 1002 ofthe actuator ring 102 a, the actuating interval associated with thefluid distribution feature 1010-6 can extend over approximately 12° ofthe circumference of the hub 1002 of the actuator ring 102 a and theactuating interval associated with the fluid distribution feature 1010-7can extend over approximately 16° of the circumference of the hub 1002of the actuator ring 102 a in the example provided).

Accordingly, when the clutch actuator 48 a (FIG. 18) outputs a minimumcompressive clutch engagement force onto the clutch pack 64 a, thereaction ring 100 a and the actuator ring 102 a can be oriented as shownin FIG. 20. In this condition, fluid exiting the supply ports 144 a-4through 144 a-7 is directed into return chambers 134 a-4 through 134a-7, respectively, and as such, the fluid does not exert a force ontothe actuator ring 102 a that would cause the actuator ring 102 a torotate in an actuating direction (i.e., a rotational direction thattends to increase the compressive clutch engagement force that is outputby the clutch actuator 48 a onto the clutch pack 64 a). Consequently,only the pressurized fluid in actuation chambers 132 a-1 through 132 a-3is employed to cause the actuator ring 102 a to rotate in the actuatingdirection. Moreover, the fluid distribution features 1010 permit fluidcommunication between the actuation chambers 132 a-4 through 132 a-7 andreturn chambers 134 a-3 through 134 a-6, respectively, so as toselectively permit relatively low-pressure fluid to enter or exit theactuation chambers 132 a-4 through 132 a-7 as the actuator ring 102 a isrotated.

When the actuator ring 102 a has rotated through an angle that exceedsthe actuating angle associated with one of the fluid distributionfeatures 1010, the seal member 1004 that is housed in a correspondingone of the projecting lugs 112 a can sealingly engage the hub 1002 ofactuator ring 102 a and thereby inhibit fluid communication betweencorresponding actuation chambers and return chambers (e.g., actuationchamber 132 a-4 and return chamber 134 a-3). Simultaneously, acorresponding separator plate (e.g., separator plate 118 a-4) can bepositioned relative to the supply port (e.g., supply port 144 a-4) sothat pressurized fluid is directed into a corresponding actuationchamber (e.g., actuation chamber 132 a-4) rather than to thecorresponding return chamber (e.g., return chamber 134 a-4). In thisway, the amount of fluid pressure that acts on the actuator ring 102 acan be varied based upon the rotational position of the actuator ring102 a relative to the reaction ring 100 a.

Configuration of the clutch actuator 48 a in this manner can beadvantageous in that the torsional output of the clutch actuator 48 amay be tailored in a desired manner without significant changes in oneor more of the flow characteristics (e.g., the mass flow rate, velocity,pressure) of the pressurized fluid.

While the invention has been described in the specification andillustrated in the drawings with reference to various embodiments, itwill be understood by those of ordinary skill in the art that variouschanges may be made and equivalents may be substituted for elementsthereof without departing from the scope of the invention as defined inthe claims. Furthermore, the mixing and matching of features, elementsand/or functions between various embodiments is expressly contemplatedherein so that one of ordinary skill in the art would appreciate fromthis disclosure that features, elements and/or functions of oneembodiment may be incorporated into another embodiment as appropriate,unless described otherwise, above. Moreover, many modifications may bemade to adapt a particular situation or material to the teachings of theinvention without departing from the essential scope thereof. Therefore,it is intended that the invention not be limited to the particularembodiment illustrated by the drawings and described in thespecification as the best mode presently contemplated for carrying outthis invention, but that the invention will include any embodimentsfalling within the foregoing description and the appended claims.

1. A power transmission device comprising: a rotary input member adaptedto receive drive torque from a source of drive torque; a rotary outputmember adapted to transmit drive torque to an output device; a torquetransmission mechanism operable for transferring drive torque from saidinput member to said output member, said torque transmission mechanismincluding a friction clutch operably disposed between said input memberand said output member and an electrohydraulic control system forcontrolling engagement of said friction clutch, said electrohydrauliccontrol system including a clutch actuator, a fluid pump, a motordriving said fluid pump, and a controller, said clutch actuatorincluding a rotary operator and a thrust mechanism, said rotary operatorhaving first and second components with a plurality of actuationchambers and a plurality of return chambers, said first component beingfixed for rotation with one of said input and output members and saidsecond component adapted to rotate relative to said first component, andsaid thrust mechanism being operable for applying a clutch engagementforce on said friction clutch in response to rotation of said secondcomponent relative to said first component, said controller beingoperable for controlling the fluid pressure supplied to at least one ofsaid actuation chambers and said return chambers to thereby controlrotation of said second component relative to said first component ofsaid rotary operator; wherein said electrohydraulic control system isconfigured so that said actuation chambers are divided into at least twogroups, wherein a first group is configured such that said activationchamber or activation chambers included therein contain a relativelyhigh pressure fluid when said second component is rotated relative tosaid first component in an actuating direction regardless of a relativeposition of said first and second components and wherein a second groupis configured such that said activation chamber or activation chambersincluded therein contain a relatively low pressure fluid when saidsecond component is positioned relative to said first component within apredetermined actuating interval and contain said relatively highpressure fluid when said second component is rotated relative to saidfirst component in said actuation direction by an amount that exceedssaid predetermined actuating interval.
 2. The power transmission deviceof claim 1, wherein each activation chamber of said second group iscoupled in fluid connection to a corresponding return chamber via afluid conduit when said second component is positioned relative to saidfirst component within said predetermined actuating interval.
 3. Thepower transmission device of claim 2, wherein said fluid conduit isformed between said first and second components.
 4. The powertransmission device of claim 3, wherein said fluid conduit includes apassage that is formed in one of said first and second components. 5.The power transmission device of claim 4, wherein a seal member iscarried by said other one of said first and second components, said sealmember being sealingly engaged to said one of said first and secondcomponents and inhibiting fluid communication between said actuationchamber of said second group and said corresponding return chamber whensaid second component is positioned relative to said first component byan amount that exceeds said predetermined actuating interval.
 6. Thepower transmission device of claim 1, wherein each of said activationchambers is associated with a distinct supply port and said supply portof each activation chamber in said second group is shifted relative tosaid supply ports of said activation chambers of said first group by apredetermined offset interval.
 7. The power transmission device of claim6, wherein the predetermined offset interval is about equal to thepredetermined actuating interval.
 8. The power transmission device ofclaim 1, wherein the electrohydraulic control system is configured toinclude a third group of activation chambers, the third group isconfigured such that said activation chamber or activation chambersincluded therein contain a relatively low pressure fluid when saidsecond component is positioned relative to said first component within asecond predetermined actuating interval and contain said relatively highpressure fluid when said second component is rotated relative to saidfirst component in said actuation direction by an amount that exceedssaid second predetermined actuating interval.
 9. The power transmissiondevice of claim 8, wherein the second predetermined actuating intervalis greater than the first predetermined actuating interval.
 10. Thepower transmission device of claim 8, wherein each of said activationchambers is associated with a distinct supply port and said supply portof each activation chamber in said third group is shifted relative tosaid supply ports of said activation chambers of said first group by apredetermined offset interval.
 11. The power transmission device ofclaim 10, wherein the predetermined offset interval is about equal tothe second predetermined actuating interval.
 12. The power transmissiondevice of claim 8, wherein the electrohydraulic control system isconfigured to include a fourth group of activation chambers, the fourthgroup being configured such that said activation chamber or activationchambers included therein contain a relatively low pressure fluid whensaid second component is positioned relative to said first componentwithin a third predetermined actuating interval and contain saidrelatively high pressure fluid when said second component is rotatedrelative to said first component in said actuation direction by anamount that exceeds said third predetermined actuating interval.
 13. Thepower transmission device of claim 12, wherein the third predeterminedactuating interval is greater than the second predetermined actuatinginterval.
 14. The power transmission device of claim 12, wherein each ofsaid activation chambers is associated with a distinct supply port andsaid supply port of each activation chamber in said fourth group isshifted relative to said supply ports of said activation chambers ofsaid first group by a predetermined offset interval.
 15. The powertransmission device of claim 14, wherein the predetermined offsetinterval is about equal to the third predetermined actuating interval.16. The power transmission device of claim 12, wherein theelectrohydraulic control system is configured to include a fifth groupof activation chambers, the fifth group being configured such that saidactivation chamber or activation chambers included therein contain arelatively low pressure fluid when said second component is positionedrelative to said first component within a fourth predetermined actuatinginterval and contain said relatively high pressure fluid when saidsecond component is rotated relative to said first component in saidactuation direction by an amount that exceeds said fourth predeterminedactuating interval.
 17. The power transmission device of claim 16,wherein the fourth predetermined actuating interval is greater than thethird predetermined actuating interval.
 18. The power transmissiondevice of claim 16, wherein each of said activation chambers isassociated with a distinct supply port and said supply port of eachactivation chamber in said fifth group is shifted relative to saidsupply ports of said activation chambers of said first group by apredetermined offset interval.
 19. The power transmission device ofclaim 18, wherein the predetermined offset interval is about equal tothe fourth predetermined actuating interval.
 20. The power transmissiondevice of claim 1, wherein said first component of said rotary operatoris a reaction ring having a cylindrical body segment and a plurality ofradially extending first lugs which define a series of channelstherebetween, wherein said second component is an actuator ring having acylindrical body segment and a plurality of radially extending secondlugs which extend into said channels so as to define a plurality of saidactuation chambers and return chambers, wherein said actuator chambersare in fluid communication with an outlet of said control valve, andwherein said fluid pump is operable to draw fluid from a fluid sourceand deliver high pressure fluid to said control valve such thatselective control of said control valve results in rotary movement ofsaid actuator ring relative to said reaction ring.
 21. The powertransfer device of claim 20, wherein said actuator ring is fixed to adrive component of said thrust mechanism such that rotation of saiddrive component results in translational movement of a driven componentof said thrust mechanism for exerting said clutch engagement force onsaid friction clutch.
 22. The power transfer device of claim 21, whereinsaid thrust mechanism is a ball ramp unit with a first cam member as itsdrive component, a second cam member as its driven component, androllers retained in cam tracks formed between said first and second cammember, and wherein said cam tracks are configured to causetranslational movement of said second cam member in response to rotarymovement of said first cam member for applying said clutch engagementforce to said friction clutch.
 23. A power transfer device for use in amotor vehicle having a powertrain and first and second drivelines,comprising: a first shaft driven by the powertrain and adapted forconnection to the first driveline; a second shaft adapted for connectionto the second driveline; a torque transmission mechanism fortransferring drive torque from said first shaft to said second shaft,said torque transmission mechanism including a friction clutch operablydisposed between said first shaft and said second shaft, a clutchactuator for engaging said friction clutch and a control system foroperating said clutch actuator, said clutch actuator including a fluidpump, a rotary operator and a thrust mechanism, said rotary operatorincluding first and second components which define a plurality ofactuation chambers and a plurality of return chamber that are adapted toreceive pressurized fluid from said pump, said first component beingfixed for rotation with one of said first and second shafts and saidsecond component adapted to rotate relative to said first component inresponse to the fluid pressure in said actuation and return chambers,and said thrust mechanism is operable for applying a clutch engagementforce to said friction clutch in response to rotation of said secondcomponent relative to said first component, said control systemincluding a motor driving said pump and a controller for controllingactuation of said motor and regulating the fluid pressure supplied to atleast one of said actuation chambers and said return chambers; whereinsaid actuation chambers are segregated into a plurality of groups, afirst one of the groups being configured such that said activationchamber or activation chambers included therein contains a relativelyhigh pressure fluid when said second component is rotated relative tosaid first component in an actuating direction regardless of a relativeposition of said first and second components and wherein each of theother groups is configured such that said activation chamber oractivation chambers included therein contain a relatively low pressurefluid when said second component is positioned relative to said firstcomponent within a corresponding actuating interval and contain saidrelatively high pressure fluid when said second component is rotatedrelative to said first component in said actuation direction by anamount that exceeds said corresponding actuating interval, and whereineach of the corresponding actuating intervals is different so that aquantity of said actuating intervals that are employed to exert a forceonto said first and second components to cause said second component tobe rotated relative to said first component in said actuation directionis dependent upon said relative position of said first and secondcomponents.
 24. The power transfer device of claim 23, wherein each ofsaid activation chambers is associated with a distinct supply port andsaid supply port of each activation chamber in each of said other groupsis shifted relative to said supply ports of said activation chambers ofsaid first group an offset interval that is unique to an associated oneof said other groups.
 25. The power transfer device of claim 24, whereinthe offset interval of each of said other groups is about equal to saidcorresponding actuating interval of said other group.
 26. The powertransfer device of claim 23, wherein each activation chamber of saidother groups is coupled in fluid connection via a fluid conduit to acorresponding return chamber when said second component is positionedrelative to said first component within a smallest one of saidcorresponding actuating intervals.
 27. The power transfer device ofclaim 26, wherein said fluid conduit is formed between said first andsecond components.
 28. The power transfer device of claim 27, whereinsaid fluid conduit includes a passage that is formed in one of saidfirst and second components.
 29. The power transfer device of claim 23,wherein said first component of said rotary operator is a reaction ringhaving a cylindrical body segment and a plurality of radially extendingfirst lugs which define a series of channels therebetween, wherein saidsecond component is an actuator ring having a cylindrical body segmentand a plurality of radially extending second lugs which extend into saidchannels so as to define a plurality of said actuation chambers andreturn chambers, wherein said actuator chambers are in fluidcommunication with an outlet of said control valve, and wherein saidfluid pump is operable to draw fluid from a fluid source and deliverhigh pressure fluid to said control valve such that selective control ofsaid control valve results in rotary movement of said actuator ringrelative to said reaction ring.
 30. The power transfer device of claim29, wherein said actuator ring is fixed to a drive component of saidthrust mechanism such that rotation of said drive component results intranslational movement of a driven component of said thrust mechanismfor exerting said clutch engagement force on said friction clutch. 31.The power transfer device of claim 30, wherein said thrust mechanism isa ball ramp unit with a first cam member as its drive component, asecond cam member as its driven component, and rollers retained in camtracks formed between said first and second cam member, and wherein saidcam tracks are configured to cause translational movement of said secondcam member in response to rotary movement of said first cam member forapplying said clutch engagement force to said friction clutch.
 32. Atorque transmission mechanism for use in a motor vehicle having apowertrain and a driveline, comprising: an input member driven by thepowertrain; an output member driving the driveline; a clutch packoperably disposed between said input and output members; an apply platemoveable relative to said clutch pack between a first position and asecond position, said apply plate is operable in its first position toapply a minimum clutch engagement force on said clutch pack and saidapply plate is operable in its second position to apply a maximum clutchengagement force on said clutch pack; a clutch actuator for controllingmovement of said apply plate between its first and second positions,said clutch actuator including a fluid pump, a rotary actuator and athrust mechanism, said rotary operator having first and secondcomponents that are coaxially arranged to define an actuation chamberand a return chamber therebetween which are adapted to receivepressurized fluid from said pump, said first component of said rotaryoperator is fixed for rotation with one of said input and output membersand said second component is adapted to rotate relative to said firstcomponent in response to the fluid pressure in said actuation and returnchambers, and said thrust mechanism is operable to move said apply platebetween its first and second positions in response to rotation of saidsecond component relative to said first component; and a control systemincluding a controller for regulating the fluid pressure supplied to atleast one of said actuation and return chambers; wherein said first andsecond components cooperate to define a plurality of hydraulic valves,each of said hydraulic valves being configured to relieve fluid pressurein an associated one of said actuation chambers when a position of saidsecond component relative to said first component is within anassociated actuating interval.
 33. The power transmission device ofclaim 32, wherein each associated actuating interval is different. 34.The power transmission device of claim 32, wherein each hydraulic valveincludes a hydraulic conduit that is configured to transmit fluidbetween one of said actuating chambers and an associated one of saidreturn chambers when said position of said second component relative tosaid first component is within said associated actuating interval. 35.The power transmission device of claim 34, wherein said fluid conduitincludes a groove that is formed in one of said first and secondcomponents.
 36. The power transmission device of claim 34, wherein eachhydraulic valve includes a supply port and wherein a location of atleast two of said supply ports relative to a datum is staggered so thatrelatively high pressurize fluid is directed into said actuatingchambers at different times depending on a position of said secondcomponent relative to said first component.